Carbon containing anodes used in the electrolytic production of metals, such as aluminum, must have special properties to withstand the operating conditions. Thus, one problem exists because the molten halide electrolytes generally used are at temperatures of 700.degree. C. or higher providing significant temperature gradient problems and thermal shock on startup and because of the very short distance transition zone between cooler temperatures and the molten electrolyte. Another problem exists because of the very high current flow, which can exceed 10,000 amperes, and which induces corresponding high electric field stresses which can cause an electrode to crack or crumble. These problems are compounded because the anodes in the aforesaid environmental conditions are consumed in the electrolytic process and thus must have carefully controlled physical and chemical characteristics. For example, any water therein would cause crumbling, etc. The problems are even more acute when special anode mixtures such as Al.sub.2 O.sub.3 and carbon are used as in some processes for producing aluminum, because of higher electrical resistivities and different chemical bonding relationships in the anode and chemical consumption processes in the production of the aluminum, etc. metal. Accordingly, the manufacture of all these electrodes is critical.
Generally stated two classes of electrolytic anodes for production of aluminum have been used, namely the pre-baked or precured class and self-baking or self-curing class sometimes known as Soderburg anodes. This invention is directed to the pre-baked type of anodes.
The most relevant prior art techniques representative of the state of the art prior to this invention is related in B. L. Bailey--U.S. Pat. Nos. 2,582,764--Jan. 15, 1952. Thus, screened uncalcined petroleum cokes are mixed at a critical elevated temperature of 150.degree.-325.degree. F. with an aromatic compound liquid plasticizer such as nitrobenzene or furfural with or without added pitch or tar and formed into an electrode by molding at a pressure about 2,000 psi. Then the formed anodes are fired in a furnace at 1740.degree. F. for eight days, removed and graphitized at 5070.degree. F.
As shown in C. B. Willmore--British Pat. No. 510,256 the "green" electrodes need be supported in coke dust, etc. during baking to prevent slumping or changes in shape even when forming pressures up to 15,000 psi are used. In this patent the initial cold shaping and pressure steps are undertaken without the application of heat. Thus there is a problem in the retention of shape and prevention of slumping during the baking step.
One problem during the baking step is the tendency of the anode to fracture or develop cracks. One of the reasons for this is that the vapors released from the binders mixed in the anode need to escape. This problem is discussed in J. Walker--U.S. Pat. No. 2,822,328--Feb. 4, 1958 as related to self-baking electrodes. However, in pre-baked electrodes, the escape of volatile vapors can lead to rejects because of cracks or loss of density, etc. High density and high physical strength are necessary properties of these electrodes.
The most serious problem of all is the energy consumption required in the manufacture of the electrodes. Higher energy costs and shortages of fossil fuels has changed the requirements of the industry to a search for more energy efficient methods. As may be seen from the hereinbefore recited typical prior art manufacture of carbon containing electrodes, baking cycles of eight or more days and up to twenty-one days in practice are required in ovens reaching temperatures of the order of 2000.degree. F. Other energy is required in pre-bake or heat cycles, etc. It is evident that any substantial decrease in the energy required in such prior art manufacturing techniques would constitute a significant advance of the state of the art, provided of course, that the critical characteristics of the electrodes for use in the demanding environments of molten halide salt electrolytes, and the like, may be retained.
Thus it is an object of this invention to resolve the aforesaid problems in the art and to provide improved energy saving techniques for the manufacture of carbon containing electrodes suitable for critical use in the electrolytic production of metals, such as aluminum.
Other objects, features and advantages of the invention will be found throughout the following description, drawings and claims.